This invention relates generally to the field of packaging and sealing techniques for integrated circuits. More particularly, the invention relates to a surface mountable IC package for an integrated circuit which incorporates a flexible circuit in a pre-formed chip carrier cavity package in which the flexible circuit communicates directly between the integrated circuit within the carrier package and an external circuit.
Packaging and sealing techniques for integrated and hybrid circuits are well known. Generally, an integrated circuit (IC) refers to a single chip or die device which requires only a small cavity or space for packaging the chip. Integrated circuits must be enclosed or packaged for most commercial applications to protect the IC and to connect it into an external circuit. It is common to mount these dies in small plastic or ceramic packages which must somehow be sealed to isolate the die from the atmosphere, moisture, and other contamination which could destroy the circuit or affect its operation. Obviously it is desirable to produce and seal IC packages in high volume at low cost, while maintaining an effectively protected microcircuit.
There are presently three basic types of surface mountable IC packages: post-molded plastic packages, ceramic packages, and pre-molded plastic packages In most post-molded plastic packages the die is first attached to a metal leadframe and all electrical connections made before the assembly is totally encapsulated with a transfer molding compound, usually an epoxy. This is an inexpensive way to package large quantities of IC's and therefore is presently used for over 80% of all IC packages produced worldwide. However, this method has not been found to be suitable for large dies having high lead counts due to the compressive stresses that occur when the molding compound expands or shrinks due to temperature.
Ceramic packages, usually composed of alumina, consist of a ceramic base and lid which, after firing at above 400.degree. C., form a sealed cavity to hold the die as well as the metal leads which pass through the sides or base of the cavity to connect the die to an external circuit These packages are excellent for many applications, but the use of them for four-sided high lead count packages has been greatly restricted due to their cost which currently is over $6.00 per package.
Pre-molded plastic packages also consist of a base and a lid which provide a cavity to hold the die. Hence they resemble ceramic packages but are much cheaper due to less expensive materials and because their sealing and assembly is done at relatively low temperatures of less than about 200.degree. C. There are several different types of such packages, which are generally called plastic chip carriers, now being used. Their usage is increasing because they do not require large capital investments by the IC manufacturer and they permit low stress packaging of large dies with high lead counts.
Pre-formed chip carriers which can be surface mounted are particularly desirable for modern manufacturing purposes and FIG. 4 discloses two of the present standard types of such carriers. FIG. 4A shows a "leadless" plastic chip carrier, or PCC 10 comprising a base 12, lid 18 forming therebetween a cavity for housing a die 16 attached to the base 12 by adhesive 34. An adhesive layer 32 seals the base to the lid. Copper circuitry 28 is usually formed in PCC's by electrodepositing copper lines directly on the surface of a plastic base 12 by known direct plating methods. This package is referred to as "leadless" because no metal wires or other leads extend outward from the base 12. The major disadvantage of such a device is that it is unable to adequately dissipate stresses at the connect points 40 on the solder joints 42 which are caused by thermal expansion differences between the package 10 and the mounting surface 24 at the bonding pads 44.
FIG. 4B shows a plastic leaded chip carrier (PLCC). Conventionally, PLCC's have been made by stamping or etching a copper lead frame to make a circuit 29, extending the lead wires out through the adhesive seal 32 between the plastic lid 18 and base 12 and then bending the extended copper leads into a "J" shape to form the solder connect points 40 under the base 12. The major advantage of PLCC's over leadless PCC's is that the copper J-leads are flexible and can accomodate expansion differences between the PLCC and the mounting surface 24 without cracking or stressing the solder joints 42. Hence, PLCC's are now preferred, particularly for packages having 80 or more external leads. With this increasing usage of PLCC's, the industry has found it necessary to standardize package dimensions so that each manufacturer's PLCC will fit precisely on the mounting surface. JEDEC (Joint Electron Device Engineering Council) has set the standards for PLCC's with the most critical dimensions being shown as D and D.sub.2 in FIG. 4B. D.sub.2 must correspond exactly with the distance between the solder pads on the circuit board shown in FIGS. 2 and 4B. D and D.sub.2 depend on lead count and lead pitch. Presently, JEDEC Document MO-047 AA-AH specifies that D.sub.2 = 0.910" +/- 0.020" for PLCC's with 68 outer leads on 50 mil centers and D.sub.2 = 1.110" +/- 0/020" for PLCC's with 84 outer leads on 50 mil pitch, while D = 0.990 +/- 0.005 and 1.190 +/- 0.005 respectively.
It will be appreciated that a problem in making either of the surface mounted pre-molded packages shown in FIG. 4 has been in the methods utilized to bring the electrical signals from the IC chip to the bond pads on the surface of the circuit board to which the chip is to be attached. One conventional method of so doing is to use a metal lead frame sandwiched between a planar base and the cover as in FIG. 4B. While the use of stamped or etched metal lead frames is acceptable for relatively low lead count chips, as the number of leads has increased and the lead thickness has decreased with increasing circuit complexity, there has developed a need for a simpler, more reliable, less expensive construction method and for more rugged outer leads which will retain their planarity and pitch during production and shipment Another method of providing electrical communication between the IC chip and the printed circuit board in pre-molded packages is to electrodeposit a circuit on a rigid plastic base and then seal the base with a lid as in FIG. 4A. However, this method has been found to give low yields when used for high lead counts of 80 or more per chip, and can only produce the leadless PCC of FIG. 4A.